1. Field of the Invention
This ivention relates generally to electrochemical cells and, more particularly, this invention relates to electrodes and methods of formation thereof.
2. Description of Related Art
Electrochemical cells utilizing bipolar electrode designs having reactive metal electrodes supported on a substrate current collector are well known. See, for example, Momyer et al, U.S. Pat. No. 4,269,907 (May 26, 1981), the disclosure of which is hereby incorporated by reference, wherein cells including an aqueous electrolyte, an anode of an alkali metal, such as lithium, for example, a cathode spaced from the anode, and an intercell electrical connector are disclosed. In such cells, the cathode may comprise an electrochemically active material, such as silver oxide, and the electrolyte may comprise an aqueous alkaline solution.
Momyer et al also discloses an electrochemical cell stack comprising a plurality of bipolar electrodes connected in series.
The preparation of bipolar electrodes wherein a cathode and an anode are disposed on opposite sides of an electrically conducting metallic substrate typically involves the oxidation/reduction of a precursor electrode material. For example, the preparation of a bipolar electrode having a silver oxide cathode typically involves oxidation of elemental silver. Typically, the elemental silver is sintered and then hot forged onto a substrate current collector. Nickel foil plated with silver, so as to facilitate adherence of elemental silver thereto, is commonly used as the substrate current collector.
In the oxidation of such precursor battery electrodes the hot forgings are assembled into a stack in which the elemental silver electrodes and counterelectrodes comprising a second kind of nickel foil are alternated, with the elemental silver electrodes in the charging stack electrically connected in parallel for attachment to the positive post of a DC power supply. Further, all the nickel foil counterelectrodes are electrically connected in parallel for attachment to the negative post of the aforementioned DC power supply. The stack is then placed into an electrolyte solution, permitting electrical contact between the electrodes.
In principle, no precursor electrode will exhibit a voltage rise independent of the other precursor electrodes because each of the precursor electrodes is made electrically common. Thus, when one of the precursor electrodes completes oxidation prior to the others, then even an infinitesimal increase in voltage produces an increased back electromotive force (EMF) which results in a drop-off in current through the already oxidized electrode and an altering of the current path through the other electrodes and thus a different current sharing pattern therein.
In addition, the conventional electrode formation technique of parallel oxidation is frequently accompanied by a bending of the electrodes. For example, the silver oxide electrodes resulting from the use of the above-identified method of oxidation are frequently of a bent, irregular shape. The bending of the electrode is believed to be largely a result of the stoichiometric and molar volume changes which occur upon oxidation during electrode formation and is commonly referred to as "potato chipping".
There are in addition two other problems caused by application of prior art techniques to bipolar electrode configurations. One is that the nickel foil at the anode potential oxides which hinders adhesion of the anode metal, e.g., lithium or aluminum. The second is that the parasitic oxidation decreases charging efficiency and for example, in the case of silver precursor electrodes, masks the voltage rise associated with the oxidation of silver to the divalent state, resulting in a low capacity of about 15 ampere-minutes per gram in the case of silver oxide.